The present invention relates to the separation of solids from liquids by filtration through granular media and, more particularly, to an improvement in apparatus for granular media filters of the type having a tank and a bed of granular filter media in the tank through which liquids having suspended or dissolved solids therein flow for removing the solids to clean the liquid.
Granular media filters are known for removing solids from liquids. Such filters have long been utilized for potable water treatment but have only recently been introduced to wastewater treatment plants, where the solids may comprise microbial flocs, coagulant residues, and a variety of other relatively unpredictable substances. Such filters normally include a tank for receiving flowing liquid containing suspended solids, a bed of granular media supported within the tank, means for removing liquid which has passed through the media (filtrate), and means for periodically washing the granular media to remove particulates collected therein during filtration. Because the washing step is usually accomplished by passing liquid through the bed in a direction opposite to the flow direction for filtration, the washing step is usually referred to as backwashing.
In one well-known granular media filter construction, filtration is accomplished in the downward direction so that filtrate is removed from beneath the media. Such "downflow" filters may be capable of either semi-continuous or continuous operation. In this latter operation, partitioning walls are fixedly mounted to form a plurality of individual cells within the granular media bed so that backwashing can be accomplished in one of the cells while filtration proceeds in the other cells. Such filtering machines are shown in U.S. Pat. Nos. 3,239,061 and 4,151,265.
In "semi-continuous" granular media filters, the entire granular bed is used simultaneously for filtration until the bed collects solids to the extent that its resistance to flow adversely affects the rate of operation of the machine, or the effectiveness of removing solids. Then the machine is removed from filtration service and the entire bed is cleaned as a unit.
Granular media filters may also be of the type used to remove dissolved solids from liquid. Such filters typically use filter media of activated carbon, which physically adsorbs dissolved solids from the liquid. Over time, the activated carbon may adsorb so much dissolved solids material as to become saturated or spent. In this event, the spent filter media must be removed from the filter, and new or regenerated media delivered to the filter. Similiarly, in granular media filters of the above-described type used to remove suspended solids, the media may after long usage be rendered unsuitable for further filtering, even if backwashed. This media must then be removed, and new media or media cleaned by methods other than backwashing must be delivered to the filter. With either type of filter, the filtration operation of the filter must be terminated while the filter media is being replaced.